If you lived anywhere along the Washington-New York corridor this February, you might have sworn you were somewhere else. The normally expressionless, brown landscape was uncommonly white, snow plows could be heard scraping the streets at all hours of the night, and those hiking boots collecting dust in the back of the closet had to be brought out simply to make the trip to the corner deli. In Baltimore, more snow fell in February than in any other month in recorded history. In the nation’s capital, the month proved to be the coldest February in a quarter century. And in Philadelphia and New York, the President’s Day storm of 2003 went down as one of the snowiest of all time.

Meanwhile, 5,000 miles away in the equatorial Pacific, yet another El Niño was brewing. Trade winds across the Pacific had broken up and warm waters from the western Pacific had crept eastward. The natural thing to do, of course, would be to blame El Niño for the harsh winter conditions in the mid-Atlantic States. And why not? In the past, everything from fish deaths in the Pacific, high rainfall along the West Coast, tornadoes in the Midwest, and hurricanes off of Central America have been tied to the periodic Pacific Ocean warming. This year, however, our suspicions might at least be partially misplaced. Many climate experts are maintaining that the snows in the mid-Atlantic region may be due in part to yet another, lesser known, cyclical climate anomaly called the North Atlantic Oscillation (NAO).

Despite the lack of publicity the NAO receives, its impact can be felt every winter around the entire Atlantic basin. Unlike El Niño, which occurs cyclically every three to seven years, the NAO appears to fluctuate randomly on a yearly basis. There are, however, slow variations in the NAO that point to an influence outside the atmosphere.

A strong North Atlantic Oscillation (large difference
in pressure between the mid-latitude and tropical North Atlantic) tends to produce more severe weather
in the North Atlantic, increased snowfall in Sweden (left), and an early spring in
Washington, DC (right) [Photographs copyright Mark Schoeberl (left) & Barbara Summey (right)]

Scientists around the world have been studying the NAO to understand what drives its variations and perhaps even to someday forecast its behavior. It is an elusive goal because of the NAO’s arrhythmic behavior. Researchers have had modest success, however, with short-term forecasts by linking the anomaly to sea-surface temperatures through statistical correlation. And over a longer time scale, they have discovered that global warming may be slowly influencing the strength of the NAO and its effect on the climate.

Typical weather caused by a negative NAO (small pressure difference between the mid-latitude and tropical North Atlantic) is fewer storms overall, clear skies over the Normandy coast of France (left,) and rain in Spain (right). (Photographs copyright Eric Simmon)

The Highs and Lows of the NAO

For those who still don't know what those "Hs" and "Ls"
stand for on the nightly weather report, air pressure is a measure of how much
air is pushing down on the surface of the Earth at a given point. Generally,
high- and low-pressure systems form when air mass and temperature differences
between the surface of the Earth and the upper atmosphere create vertical
currents. In a low-pressure system, these vertical winds travel upwards and suck
air away from the surface of the Earth like a giant vacuum cleaner, decreasing
the air pressure above the ground or sea. This decrease in surface air pressure
in turn causes atmospheric currents moving parallel to the surface of
the Earth near the base of the low to spin counter clockwise (clockwise in the
Southern Hemisphere). Conversely, in a high-pressure
system, air is being pushed down on the ground like a vacuum put in reverse. The
downward vertical winds cause an increase in air pressure on the ground and force atmospheric currents to spin clockwise (counter clockwise in the
Southern Hemisphere). Both lows and highs function
like giant slow-moving hurricanes and anti-cyclones, respectively. The higher in
pressure a high-pressure system gets or the lower in pressure a low-pressure
system gets, the more robust and larger this spinning circulation pattern
becomes.

A low pressure system will
pull in air from the surrounding area. Winds around a low spiral counter-clockwise (in the Northern Hemisphere,
clockwise in the Southern Hemisphere) and upwards towards the center of the system. View animation [945kb] (Image by Robert Simmon)

Air is pushed away from a high pressure
system. The winds rotate clockwise (in the Northern Hemisphere, counter-clockwise in the Southern Hemisphere) and
away from the system's center. View animation [910kb] (Image by Robert Simmon)

“Generally speaking the NAO is an oscillation in atmospheric mass between a low around Greenland and Iceland and a high over the Azores west of Portugal,” says Vikram Mehta. He is an atmospheric scientist at NASA’s Goddard Space Flight Center who has been studying Atlantic climate anomalies for over 10 years.

He explains that a permanent low-pressure system exists over Greenland and Iceland, and a permanent high-pressure system exists over a group of islands roughly 900 miles (1400 kilometers) west of Portugal, known as the Azores. For most of the year, the high and the low are mild, and their influence on the Atlantic basin climate is minimal. When winter hits, however, all of this begins to change. Both pressure systems grow much more intense and begin to fluctuate from week to week between two different states. In one state, which scientists call a positive NAO, the high-pressure system grows especially high, while the low-pressure system grows especially low, creating a large pressure difference between the Azores and Iceland. In the other state, known as a negative NAO, the high-pressure system weakens and the low becomes shallow, creating a milder pressure difference between the two regions of the Atlantic. As the low and high intensify and relax, the winds revolving around their centers increase and decrease in both strength and in extent. During a strong positive NAO, the two pressure systems can just about cause all the currents in the northern half of the northern Atlantic to spin counterclockwise and all those currents in the southern half to spin clockwise.

Though the impact of the NAO and its phases can be felt across the entire Atlantic and the surrounding continents, its greatest effect is on the storms passing into Europe. Between the two swirling, clockwise and counterclockwise circulation patterns created by the high and low, there is an area where they come together and form a steady, forward-moving current that channels weather systems from the United States to Europe.

“As the pressure systems vary, they modulate the winds along this track and change the number of storms and the amount of moisture over Europe coming from the Atlantic and the Gulf Stream,” says Mehta. Like two wheels of a printing press, the high and low systems can increase or decrease the strength of the winds along this channel. When the pressure difference between the two systems is large (a positive NAO index), the winds along this conduit pick up, and they push the storms north towards Scandinavia and northern France. When the pressure difference is small (a negative NAO index), the storms take a more direct course from the southern United States to southern Europe, the Middle East, and northern Africa.

Jim Hurrell is an atmospheric scientist at the National Center for Atmospheric Research who spent a number of years analyzing the connection between the North Atlantic Oscillation and winter weather. He says, “The direction these storms take as a result of NAO can cause remarkable changes in the temperature and the weather over Europe from December through March.” A positive NAO on average can increase rainfall in northern Europe by a little over an eighth of an inch per day and warm the air there by roughly 5 degrees Fahrenheit (2.8 degrees Celsius). If the condition persists for most of the winter, it can lengthen the growing season by 20 days in Sweden, lower reindeer populations in Norway, lead to water shortages in the Fertile Crescent, and provide sunnier, drier conditions for tourists on the French Riviera. A negative NAO, on the other hand, will bring rain to southern Europe, drop the temperatures in northern Europe, and maintain the already warm climate across the Mediterranean. If the negative state persists, it will increase the production of olives and grapes in Greece, put Denmark in a deep freeze, and create ideal skiing conditions in Austria.

The
positive and negative phases of the
North Atlantic Oscillation are defined by the differences in pressure between
the persistent low over Greenland and Iceland and the persistent high
off the coast of Portugal. During a positive NAO, both systems are stronger than
usual. That is, the low has a lower atmospheric pressure and the high has a higher
atmospheric pressure. During the negative phase of the NAO, both systems are
weaker, lowering the difference in pressure between them. (Images by Robert Simmon)

Hurrell said the NAO’s effects could also be felt to a lesser degree in the United States. When the NAO is classically positive, the high-pressure system residing near the Azores strengthens. The winds rotating around the system expand and push warm air from the tropical Atlantic and the Caribbean northward. “So on the west side of the Azores high you have warm air being advected on to the Caribbean and up onto the East Coast, creating a warmer winter along the mid-Atlantic States,” says Hurrell. The result is typically less snowfall for the Washington-New York corridor. During a negative NAO, the high-pressure system grows weak and winter storms, and cold weather that are normally meant for Boston and Maine, head south.

“As to this winter, though the NAO was slightly negative this year and has contributed to the winter weather, it does not appear to be a classic NAO pattern,” he says. He explains that the El Niño has led to a slight deepening of the low-pressure system that typically sits over the southeastern United States during the winter, which has brought colder temperatures and more precipitation to the mid-Atlantic. If the NAO was strongly positive on average, as it has been in recent years, then the warm temperatures from the Azores high would likely have counteracted these colder temperatures and the weather would have been much milder along the East Coast. But this year the NAO was neither particularly strong nor particularly weak, and the dominant low-pressure system in the Atlantic was a bit south and east of Greenland. The net effect has been the cold weather and winter storms in the northeast United States. Hurrell adds that there weren’t any pronounced impacts on the European spring or winter either and that storms have tended towards the northern and southern Europe largely without bias.

The maps at left show the
relationship between a strong positive NAO and precipitation and temperature. Positive correlation means that an area
is wetter or warmer than normal, negative correlation means an area is drier or colder than normal, and no
correlation means the area is unaffected by the NAO. (Images courtesy Lamont-Doherty Earth Observatory)

Relying on the Oceans Long Term Memory

Observing the NAO on a year to year basis, however, does not render a complete picture of how the anomaly affects the climate of the Atlantic basin. From week to week, the NAO flip-flops between positive and negative phases seemingly at random, sending good and bad weather intermittently to both southern and northern Europe. Yet, each winter the NAO almost always shows a predominantly negative or positive average for the year. When these yearly averages are put into an index and plotted next to one another, a clear pattern emerges. Since the 1960s, the entire index has overall been growing more positive.

One of the goals for atmospheric scientists studying the NAO has been to predict the sign and strength of the NAO from year to year and decade to decade. They could then warn European farmers of when they should plant their crops, alert Mediterranean resorts as to the amount of rain they are likely to receive, and generally forecast next year’s winter weather trends more accurately. Despite its regular appearance, the NAO is still too erratic to predict by looking at a chart of its history. “There are simply too many variables that go into these trends to make them easily predictable,” says Hurrell.

The only way scientists could forecast the dips and peaks of the NAO is if they first understood exactly what was causing the two pressure systems to vary relative to one another. Most agree that the high and low would develop on their own over the Atlantic and that they would change in strength from week to week or even month to month. The irregular sinusoidal pattern exhibited by the NAO, however, would require some type of climatic memory. For the NAO yearly averages to climb upward or downward over several consecutive winters, there would have to be some mechanism in the atmosphere or the ocean that keeps track of where the Azores high and the Icelandic low were the year before. But atmospheric currents change in temperature and density so rapidly over time that there is no way they could maintain a pattern into the spring and summer months after the low- and high-pressure systems break up. The current thinking is the NAO’s variation must be tied to the land or the ocean.

Several years ago scientists made a breakthrough when they confirmed through the use of computer models that part of this climatic memory driving the NAO lies in the deeper ocean temperatures of the Atlantic and changes in these temperatures are largely responsible for variations in the NAO. Mark Rodwell, a climate researcher at the Met Office in the United Kingdom, was one of the researchers who made the connection. Based on this earlier work, he is now using similar models to make forecasts on the sign of NAO nearly one year in advance.

Although the pressure difference between Iceland and Lisbon (known as the NAO index) varies from day to-day, the average index for a single winter is generally positive or negative. This graph show the daily NAO index from December 2002 through March 2003. The average for the past winter was -0.56. (Graph by Robert Simmon, based on data from the NOAA Climate Prediction Center)

“Though this is largely a statistical relationship, there is a reason behind our forecasts. The idea is that if you want to make a forecast for the winter, then you need to look at sea surface temperatures of the winter before that,” says Rodwell. The NAO is responsible for the path of strong storms that pass across the Atlantic, and these strong storms influence the temperatures of the ocean. By the spring of each year, the NAO has left a deep mark on the temperatures of the Atlantic. During the summer, these ocean temperatures are largely preserved because a relatively thin layer of water heated by the sun covers the ocean beneath like a thermal blanket. When the following winter rolls around, the warm layer is removed, revealing the sea temperatures from the previous spring, which in turn affect air pressure over the Atlantic and the next NAO.

To make their forecast, Rodwell obtains the average sea surface temperatures for the current May from satellite readings. May temperatures are the best indicators of ocean temperatures the following year, as winter storms have dwindled and the thin layer of summer water has yet to cover the ocean. Rodwell plugs these values into a computer simulation of the atmosphere and ocean over the Atlantic and runs the simulation forward through the next winter to obtain a forecast of the average values for the NAO for January through March of the following year. “Using the forecast, we’d expect to get the sign of the NAO with 66 percent accuracy. This is better than the 50 percent chance you’d have without any forecast at all,” says Rodwell. Unfortunately, this year, the model wasn’t quite on target. It predicted a slightly positive NAO to occur over the winter 2003.

Researchers predict the state of the NAO by looking at the North Atlantic’s sea surface temperature in the spring. The graph at left shows predicted NAO index in dark green and observered NAO index in light gree. Prior to 1999, the “predictions” were based on historical data. These predictions are correct about two thirds of the time. (Graph by Mark Rodwell, United Kingdom Met Office)

Mimicking Mother Nature

Though studying the short-term variations of the NAO is key to understanding the phenomenon, climatologists at present are much more excited about what has occurred to the NAO index since the late 1960s. For a little over 100 years, since people started collecting reliable pressure readings of the Atlantic, the yearly average NAO values remained within a set range—the peak negative and positive NAO values never went far above or below what they had been in years past. Then about 30 to 40 years ago the entire NAO index started to become more positive. The peak NAO years began growing more positive and the negative NAO years began growing less negative.

“It’s to the point to where over the past 20 years we’ve seen a predominantly positive North Atlantic Oscillation,” says Hurrell. The overall increase in the index has corresponded to a noticeably longer growing season in Europe and milder winters in the mid-Atlantic region of the United States. This past winter has been one of the few exceptions.

Lately, Hurrell’s research into the NAO has focused on uncovering the cause of the upward trend. Such a trend would suggest that something bigger than ocean temperatures or currents in the Atlantic, something acting on a global scale, was pushing at the NAO. For now, the prime suspect is global warming. Hurrell explains that over the past 30 years a correlation has existed between the rise in the North Atlantic Oscillation and an increase in temperatures in the Indian Ocean. Within the climatology community, it’s fairly well established that the increased temperatures and rainfall in the Indian Ocean is due directly to global warming produced by greenhouse gases.

In an attempt to verify that warming temperatures in the Indian Ocean actually caused a change in pressure differences 7,000 miles (11,000 kilometers) away over the Atlantic, Hurrell turned to a computer simulation of the Earth’s atmosphere and ocean. Using such models, researchers can explore connections between the ocean and the atmosphere that span half the globe. “When we ran our model forward [in time] and forced an increase in sea surface temperatures and precipitation over the Pacific Ocean and Indian Ocean, we saw a strong positive NAO response. The Indian Ocean appears to have a significant influence over the North Atlantic region,” says Hurrell.

Though much work still has to be done to reach the point where scientists understand this overall shift in the NAO index, Hurrell believes that in the end, long-term predictions of the NAO’s sign will likely be more feasible than year to year predictions. He explains there are simply too many random variables that influence the NAO on a yearly basis. He does, however, envision a day when scientists will be able to say with a degree of certainty how positive or negative the NAO will be in the coming decade. As to the benefits of such a prediction, he points to the Norwegians reliance on hydropower. A positive NAO brings an abundance of water and power to Norway come spring, and a negative NAO sends Norway searching for imported power. “If we can tell the Norwegians that the NAO is generally positive for the next 10 years, then they have at least some assurance they can rely on hydropower for the next decade. If not, then they can begin to make plans in advance to import electricity,” says Hurrell. “I feel some really useful things can come of this.”

From year to year, the winter NAO index fluctuates dramatically, but there are some long-term trends. Since a low point in 1960, winter NAO Indices have been rising. This is associated with mild winters in the eastern U.S. and longer growing seasons in western Europe. (Graph by Jim Hurrell, University Corporation for Atmospheric Research)

Air is pushed away from a high pressure system. The winds rotate clockwise
(in the Northern Hemisphere, counter-clockwise in the Southern Hemisphere) and
away from the system's center. (Animation by Robert Simmon)

A low pressure system will
pull in air from the surrounding area. Winds around a low spiral counter-clockwise (in the Northern Hemisphere,
clockwise in the Southern Hemisphere) and upwards towards the center of the system. (Animation by Robert Simmon)